A humidity sensor for detecting humidity of atmosphere around the sensor is disclosed in, for example, JP-A-2002-243690 corresponding to U.S. Pat. No. 6,580,600. The sensor includes a semiconductor substrate, a silicon oxide film on the substrate, a pair of electrodes disposed on the same plane of the silicon oxide film so as to separate from each other, a silicon nitride film covering the electrodes, and a humidity sensitive film disposed on the silicon nitride film. The humidity sensitive film covers the electrodes and is disposed between the electrodes. A capacitance between the electrodes is changeable in accordance with humidity of atmosphere around the sensor. Based on the capacitance, the humidity is detected by the sensor.
The humidity sensitive film is made of hygroscopic organic polymer such as poly-imide and butyric acetylcellulose. The organic polymer providing the humidity sensitive film includes numerous molecules, each of which has a long chain shape, and the numerous molecules densely intertwine with each other. When the sensor is left for a long time in high humid atmosphere, the molecular group of the organic polymer untwines gradually due to water molecules in the atmosphere. The untwined polymer can adsorb water molecules more than the intertwined polymer under the same condition. Thus, the humidity sensitive film composed of the untwined polymer can adsorb water molecules in the atmosphere more than the humidity sensitive film composed of the intertwined polymer under the same humidity condition. This phenomenon is defined as swelling phenomenon. When the humidity sensitive film adsorbs much water molecules, as shown in FIG. 14, a sensor output increases even when relative humidity is the same. Here, a curve XIVA represents a relationship between a sensor output of a humidity sensor and relative humidity when the sensor is used for a long time so that the humidity sensitive film adsorbs much water molecules. A curve XIVB represents a relationship between a sensor output of a humidity sensor and relative humidity when the sensor is not substantially used after the sensor is manufactured. Thus, the sensor output in the humidity sensor is deviated, so that detection accuracy of the relative humidity in the atmosphere is reduced. Thus, it is required for the humidity sensor to maintain the detection accuracy for a long time.